Applications of homogeneous dynamics: from number theory to statistical mechanics

Transcription

1 Applications of homogeneous dynamics: from number theory to statistical mechanics A course in ten lectures ICTP Trieste, July 2015 Jens Marklof and Andreas Strömbergsson Universities of Bristol and Uppsala supported by ERC, Göran Gustafsson Foundation, Swedish Research Council 1

2 Lecture I: Introduction 1. What is homogeneous dynamics? 2. What is measure rigidity? 3. Three stunning applications of measure rigidity (a) Margulis proof of the Oppenheim conjecture (b) Quantum chaos (c) Randomness mod 1 4. Overview of the remaining 9 chalk and talk lectures 2

3 What is homogeneous dynamics? 3

4 Homogeneous dynamics (= dynamics on a homogeneous space) Consider a homogeneous space of the form where X =Γ\G = {Γg : g G} G a Lie group Γ a lattice in G A lattice Γ in G is a discrete subgroup such that there is a fundamental domain F Γ of the (left) Γ-action on G with finite left Haar measure. This in turn implies that G is unimodular, i.e. left Haar measure = right Haar measure. 4

5 We can generate dynamics on X =Γ\G by right multiplication by a fixed element Φ G: X X, x xφ t where t =0, 1, 2, 3,...(discrete time dynamics) or t R 0 (continuous time dynamics = homogeneous flow ). Note that {Φ t } t R is a one-parameter subgroup of G. 5

6 Example: The simplest finite-volume homogeneous space G = R (viewed as the additive group of translations of R; abelian!) Γ=Z (the group of integer translations) X = T = Z\R = R/Z (= [0, 1] glued at opposite sides = circle of length one) Haar measure = Lebesgue measure = dx Φ=α (translation by α) x xφ =x + α mod 1 (rotation by α) 6

7 What is measure rigidity? 7

8 An illustration: Two proofs of a classic equidistribution theorem Theorem (Weyl, Sierpinsky, Bohl ). If α R \ Q then, for any continuous f : T R, lim N 1 N N n=1 f(nα) = T f(x) dx Hermann Weyl ( ) α = 2, N = 100 8

9 Proof #1 harmonic analysis (after Weyl 1914) By Weierstrass approximation theorem (trigonometric polynomials are dense in C(T)) it is enough to show that lim N 1 N N n=1 f(nα) = holds for finite Fourier series f of the form f(x) = K k= K To establish eq. (1) we have to check that lim N 1 N N n=1 and secondly that for every k = 0 lim N 1 N N n=1 e 2πiknα = c 0 = c 0 = lim N T c k e 2πikx. T e 2πikα N f(x) dx (1) f(x) dx (2) 1 e 2πikNα =0. (3) 1 e2πikα Eq. (2) is obvious and (3) follows from the formula for the geometric sum (which requires α/ Q). 9

10 Proof #2 measure rigidity The linear functional f ν N [f] := 1 N N n=1 f(nα) defines a Borel probability measure on T. Since T is compact, the sequence (ν N ) N is relatively compact, i.e., every subsequence contains a convergent subsequence (ν Nj ) j. Suppose ν Nj ν. What do we know about the probability measure ν? in the weak*-topology 10

11 Define the map T α : T T, x x + α. For each f C(T) we have ν N [f T α ]= 1 N = 1 N N n=1 N n=1 f((n + 1)α) f(nα)+ 1 N f((n + 1)α) 1 N f(α) and so ν Nj [f] ν[f] implies ν Nj [f T α ] ν[f]. Since f T α C(T) we also have ν Nj [f T α ] ν[f T α ] and so ν[f T α ]=ν[f] for all f C(T) 11

12 Conclusion: ν is T α -invariant, and hence also invariant under the group T α = {T n α : n Z}. If α/ Q, it is well known that given any y T there is a subsequence of integers n i such that n i α y mod 1. This implies that ν is in fact invariant under the group of all translations of T, {T y : y T}. The only probability measure invariant under this group is Lebesgue measure, i.e. ν[f] = f(x)dx. Thus the limit measure ν is unique, and therefore the full sequence must converge: ν N [f] = 1 N N n=1 f(nα) T f(x) dx. 12

13 Why measure rigidity? In the above example there is only one measure ν that is invariant under the transformation T. We call this phenomenon unique ergodicity. This is a special case of measure rigidity = invariant ergodic measures are not abundant; each such measure has a strong algebraic structure. 13

14 The modular group A rich and famous example of a homogeneous space is X =Γ\G with G =SL(n, R) (real n n matrices with determinant one) Γ=SL(n, Z) (the modular group) The volume of X =Γ\G (with respect to Haar) was first computed for general n by Minkowski. Hermann Minkowski ( ) Examples of one-parameter subgroups: diag(e λ 1 t,...,e λ nt ) t R or 1 At 0 1 t R (λ λ n = 0) (A a fixed matrix) 14

15 The space of Euclidean lattices Every Euclidean lattice L R n of covolume one can be written as L = Z n M for some M. The bijection Γ\G { lattices of covolume one} ΓM Z n M. allows us to identify the space of lattices with Γ\G. To find the inverse map, note that, for any basis b 1,...,b n of L, the matrix M = b 1. b n base change of L. is in SL(n, R); the substitution M γm, γ Γ corresponds to a 15

16 Margulis proof of the Oppenheim conjecture... was the first application of the theory of homogenous flows to a long-standing problem which had resisted attacks from analytic number theory. As we shall see, quantitative versions of the Oppenheim conjecture can be proved by means of measure rigidity. Alexander Oppenheim ( ) Gregory Margulis (*1946) 16

17 Let Q(x 1,...,x n )= n i,j=1 q ij x i x j with q ij = q ji R. We say Q is indefinite if the matrix (q ij ) has both positive and negative eigenvalues, and no zero eigenvalues. If there are p positive and q = n p negative eigenvalues, we say Q has signature (p, q). We say Q is irrational if (q ij ) is not proportional to a rational matrix. Theorem (Margulis 1987). If Q is irrational* and indefinite with n 3 then inf Q(Z n \{0}) =0. Oppenheim s original conjecture (PNAS 1929) was more cautious it assumed n 5. The assumption n 3 in the above is however necessary; consider e.g. Q(x 1,x 2 )=x 2 1 (1 + 3) 2 x 2 2. *only required when n =3or 4 17

18 Values of quadratic forms vs. orbits of homogeneous flows The key first step in Margulis proof is to translate the problem to a question in homogeneous flows. This observation goes back to a paper by Cassels and Swinnerton-Dyer (1955) and was rediscovered by Raghunatan in the mid-1970s, leading him to formulate his influential conjectures on orbit closures of unipotent flows. It is instructive to explain this first step in the case n =2. 18

19 Observation 1: Basic linear algebra shows that there is M SL(2, R) and λ = 0so that Q(x) =λq 0 (xm), Q 0 (x 1,x 2 ):=x 1 x 2. (x =(x 1,x 2 ) is viewed as a row vector) Observation 2: Q 0 (xφ t )=Q 0 (x), for all Φ t = e t 0 0 e t, t R Need to show: Given any > 0 find t > 0 so that the lattice Z 2 MΦ t contains a non-zero vector y such that Q 0 (y) <. Observation 3: The above holds if the orbit {Z 2 MΦ t } t R {ΓMΦ t } t R is dense in Γ\G... and this is where things go wrong for n =2(but works out for n 3 since then the action of the orthogonal group of Q 0 produces a dense orbit in Γ\G). 19

20 Quantitative versions of the Oppenheim conjecture Margulis in fact proved the stronger statement that for any irrational indefinite form Q in n 3 variables Q(Z n )=R. Apart from being dense, can we say more about the distribution of the values of Q? The following theorem gives the answer for forms of signature (p, q) with n 4 and p 3. Theorem (Eskin, Margulis & Mozes, Annals of Math 1998). If Q is irrational and indefinite with p 3 then for any a<b lim T #{m Z n : m <T, a<q(m) <b} vol{x R n : x <T, a<q(x) <b} =1. For signature (2,2) the statement only holds if Q is not too well approximable by rational forms (Eskin, Margulis, Mozes 2005) (which is true for almost all forms). The problem is open for signature (2,1). 20

21 Ratner s theorem (Annals of Math, 1991) The key ingredient in the previous theorem is Ratner s celebrated classification of measures that are invariant and ergodic under unipotent flows. Ratner proves that any such measure ν is supported on the orbit Γ\ΓHg 0 Γ\G for some g 0 G and some (unique) closed connected subgroup H of G such that Γ H := Γ H is a lattice in H and thus Marina Ratner (*1938) Γ\ΓH Γ H \H is an embedded homogeneous space with finite H-invariant measure ν = ν H (the Haar measure of H). Examples of Φ t : 1 t 0 1 t R e t 0 0 e t t R unipotent not 21

22 Quantum chaos 22

23 Quantum ergodicity vs. scars ϕ j = λ j ϕ j, ϕ j D =0, 0 <λ 1 <λ 2 λ 3... Numerics: A. Bäcker, TU Dresden 23

24 Random matrix conjecture (Bohigas, Giannoni & Schmit 1984) 24

25 Berry-Tabor conjecture (Berry & Tabor 1977) 25

26 Let be the Laplacian on a compact surface of constant negative curvature ϕ j = λ j ϕ j, ϕ i 2 =1 Quantum (unique) ergodicity 0 <λ 1 <λ 2 λ 3... What are possible limits of the sequence of probability measures ϕ j (z) 2 dµ(z)? We know (by microlocal analysis): Any (microlocal lift) of a limit measure must be invariant under the geodesic flow. (Doesn t help much many measures have this property!) Numerics: R. Aurich, U Ulm 26

27 Quantum (unique) ergodicity Rudnick & Sarnak (Comm Math Phys 1994) : conjecture ϕ j (z) 2 dµ(z) dµ(z) along full sequence ( quantum unique ergodicity or QUE). Shnirelman-Zelditch-Colin de Verdiere Theorem: ϕ ji (z) 2 dµ(z) dµ(z) along a density-one subsequence j i ( quantum ergodicity ); holds in much greater generality and only requires ergodicity of geodesic flow. E. Lindenstrauss (Annals Math 2006): If surface is arithmetic (of congruence type) then we have QUE. Proof uses measure rigidity of action of Hecke correspondences and geodesic flow. Anantharaman (Annals Math 2008) & with Nonnenmacher (Annals Fourier 2007): Kolmogorov-Sinai entropy of any limit is at least half the entropy of dµ(z). This means no strong scars. 27

28 Berry-Tabor conjecture: eigenvalue statistics for flat tori Let be the Laplacian on the flat torus R d /L (L =lattice of covolume 1) with eigenfunctions ϕ j = λ j ϕ j, ϕ i 2 =1 {ϕ j (x)} = {e 2πix y : y L /±} and eigenvalues (counted with multiplicity) Note {λ j } = {m 2 : m L /±} #{λ j <λ} 1 2 V d λ d/2 (λ ), V d := πd/2 Γ( d 2 + 1) To compare the statistics with a Poisson process of intensity one, rescale the spectrum by setting ξ j = 1 2 V d λ d/2 j so that #{ξ j <ξ} ξ (ξ ) 28

29 Gap distribution Assume the lattice L is generic. Is it true that for any interval [a, b] lim N #{j N : a<ξ j+1 ξ j <b} N (which is the answer for a Poisson process) = b a e s ds? gap distribution for the recaled {λ j } = {m n 2 : m, n N}, N =2, 643, 599 WE DON T KNOW HOW TO PROVE THIS! 29

30 Simpler but still not easy: Two-point statistics Is it true that for any interval [a, b] lim N #{(i, j) :i = j N, a < ξ i ξ j <b} N = b a? Eskin, Margulis & Mozes (Annals Math 2005): YES for d = 2 and under diophantine conditions on L this reduces to quantitative Oppenheim for quadratic forms of signature (2,2) VanderKam (Duke Math J 1999, CMP 2000): YES for any d and almost all L (in measure); follows idea by Sarnak for d =2 Marklof (Duke Math J 2002, Annals Math 2003): YES for {ξ j } = {V d m α d : m Z d } and any d 2, provided α R d is diophantine of type (d 1)/(d 2). The proof is different from EMM s approach. It uses theta series and Ratner s measure classification theorem. 30

31 Randomness mod 1 31

32 Gap and two-point statistics Let ξ 11 ξ 21 ξ ξ N1 ξ N2... ξ NN..... be a triangular array of numbers ξ j = ξ Nj in [0, 1] which is ordered (ξ j ξ j+1 ) and uniformly distributed, i.e. for any interval [a, b] [0, 1] #{j N : a<ξ j <b} lim = b a. N N Do these have a Poisson limit? That is, for the gaps lim N For the two point statistics lim N #{j N : a N <ξ j+1 ξ j < b N } N #{(i, j) :i = j N, N = b a N <ξ i ξ j < b N } a e s ds? = b a? 32

33 Some results for fractional parts For almost all α, ξ j = {2 j α} is Poisson for gaps and all n-point statistics (Rudnick & Zaharescu, Forum Math 2002); applies to other lacunary sequences in place of 2 n For almost all α, ξ j = {j 2 α} has Poisson pair correlation (Rudnick & Sarnak, CMP 1998); applies also to other polynomials such as ξ j = {j 3 α} etc for certain well approximable α, there are subsequences of N such that the gap statistics of ξ j = {j 2 α} both converges to Poisson and at the same time to a singular limit (Rudnick, Sarnak & Zaharescu, Inventiones 2001); algorithmic characterization of those α for which the two-point statistics is Poisson is given by Heath-Brown (Math Proc Camb Phil Soc 2010). WE DON T KNOW WHETHER EVEN THE TWO-POINT STATISTICS ARE POISSON FOR α = 2 33

34 Fractional parts of small powers 1.0 For fixed 0 <β<1, β = 1 2, the gap and two-point statistics of {n β } look Poisson numerically -NO PROOFS! β = For β = 1 2, Elkies & McMullen (Duke Math J 2004) have shown that the gap distribution exists, and derived an explicit formula which is clearly different from the exponential. Their proof uses Ratner s measure classification theorem! At the same time, the two-point function converges to the Poisson answer (El Baz, Marklof & Vinogradov, Proc AMS 2015). The proof requires upper bounds for the equidistribution of certain unipotent flows with respect to unbounded test functions

35 A great student project gaps between logs Study the distribution of gaps between the fractional parts of log n: gaps for natural base e gaps for base e 1/5 vs. exponential distribution The proof is elementary and exploits Weyl equidistribution. For details see Marklof & Strömbergsson, Bull LMS

36 Overview of the remaining lectures 36

37 2. Geometry and dynamics of SL(2, R) (a) Hyperbolic geometry, fractional linear transformations, NAK (b) Unit tangent bundle and SL(2, R), geodesic and horocycle flows, stable/unstable manifolds, NAN (c) SL(2, Z), generators, fundamental domain 3. Equidistribution (a) Ergodicity, mixing (no proofs) (b) Equidistribution of translates of horocycles from mixing (c) Equidistribution of closed horocycles via Eisenstein series 4. The space of d-dimensional lattices (a) SL(d, R) and ASL(d, R) (b) Mahler s criterion 5. Applications in distribution modulo one (a) Statistics of Farey fractions, Hall distribution (b) The three gap theorem for nα mod 1 (new proof using the space of lattices) 6. Statistics of directions and free path length 7. Siegel-Veech formula 8. Quasicrystals I 9. Quasicrystals II 10. Frobenius numbers and circulant graphs 37

38 The Lorentz gas ( Lecture 6) Arch. Neerl. (1905) Hendrik Lorentz ( ) 38

39 Quasicrystals ( Lectures 8 & 9) 39

40 Frobenius numbers and circulant graphs ( Lecture 10) 40

41 Recommended reading M. Einsiedler and T. Ward, Ergodic Theory With a View Towards Number Theory, Springer 2011 (Sections & Proof of Theorem 11.33) J. Marklof and A. Strömbergsson, The distribution of free path lengths in the periodic Lorentz gas and related lattice point problems, Annals of Mathematics 172 (2010) (Sections 1, 2 & 5) J. Marklof, Fine-scale statistics for the multidimensional Farey sequence, in: Limit Theorems in Probability, Statistics and Number Theory, Springer 2013, pp J. Marklof and A. Strömbergsson, Free path lengths in quasicrystals, Communications in Mathematical Physics 330 (2014) (Sections 1 & 2) P. Sarnak, Asymptotic behavior of periodic orbits of the horocycle flow and Eisenstein series. Comm. Pure Appl. Math. 34 (1981), no. 6, N.B. Slater, Gaps and steps for the sequence nθ mod 1. Proc. Cambridge Philos. Soc. 63 (1967)